I'd say Podzol should spread but only in the biomes it naturally generates in and if there is at least one leaf block in a 3x100x3 area above it

That's a good idea.

There are parts of biomes in which grass is unable to propagate across dirt--or at least there was once before coarse dirt was in the game, and there might still be. There could be places like that where podzol is able to propagate, while in most places it can't.